1. Field of the Invention
The present invention relates to a means for providing up-relief to a hydraulic cylinder unit and, in particular, to a means for providing up-relief to a trim cylinder unit which forms part of a trim and tilt cylinder apparatus.
2. Description of Related Art
Conventional trim and tilt systems include a unitary trim and tilt cylinder apparatus, as disclosed in U.S. Pat. No. 5,032,094, issued Jul. 16, 1991 to Sadaji Katogi, hereinafter Katogi. Katogi discloses a trim and tilt system for trimming and tilting an outboard propulsion unit on a boat. The trim and tilt system disclosed by Katogi includes a unitary trim and tilt cylinder apparatus having a tilt cylinder unit and a pair of trim cylinder units, a stern bracket adapted to be mounted on the transom of the boat, a swivel bracket for supporting the outboard propulsion unit, the swivel bracket being pivotally supported on an upper end of the stern bracket, and a hydraulic pressure circuit for actuating the tilt cylinder unit and the trim cylinder units. The trim and tilt cylinder apparatus has an upper end pivotally supported on the stern bracket and a lower end pivotally supported on the swivel bracket.
The trim and tilt system disclosed by Katogi has the disadvantage of undue stress being applied to the trim cylinder units at a point of contact of a trim rod of each of the trim cylinder units with the swivel bracket. This excessive stress is in part due to the unitary construction of the trim and tilt cylinder apparatus. As the outboard propulsion unit pivots during the trimming and tilting procedure, the angle of the trim rods of each of the trim cylinder units changes with respect to the swivel bracket. The weight of the propulsion unit is therefore applied to the trimming procedure. As a result, a considerable amount of transverse stress is applied between the trim rods and their respective trim cylinders, leading to undue wear and fatigue.
The trim and tilt system disclosed by Katogi also has the disadvantage of generating high pressures and excessive motor current draw during the trimming and tilting procedure. This generation of high hydraulic pressures and excessive motor draw is in part due to the construction of the trim cylinder units. Hydraulic fluid is pumped into the bottoms of the trim cylinders when the outboard propulsion unit is moved from a lower position to an upper position. This causes the trim rods to extend outside the trim cylinders and engage the swivel bracket, thereby pivoting the swivel bracket and outboard propulsion unit upwardly. However, if hydraulic fluid continues to be pumped into the bottoms of the trim cylinders while the trim rods are fully extended outside the trim cylinders, this will result in the generation of high pressures and excessive motor draw. The trim cylinder units disclosed by Katogi do not have a means for providing up-relief.
There is therefore a need for a new and improved trim and tilt system that reduces the stress and pressure experienced by trim cylinder units during the trimming and tilting of a marine propulsion unit.